The invention relates to an sound-monitoring device for an industrial installation to detect possible impacts of stray bodies in the installation.
In a nuclear reactor or in any industrial plant comprising a large number of members such as pipes, containers and valves in which fluids are circulating, there exists a risk of the deterioration of the structures of the installation, due to the fact that a mechanical member, such as a bolt, a washer or an element of a valve may be detached to become a stray body moving at high speed in the installation if the fluid itself is flowing at high speed.
In fact, in such installations, an account of the strains to which the parts are subjected in service, local more or less complete ruptures may occur which may develop to the point where the part is detached and drawn by the flow to become a stray body in the installation.
In nuclear reactors in particular, these rupture-causing strains may be of thermal or mechanical origin and their effect may be aggravated by corrosion.
There exist areas of the hydraulic circuits of the above-mentioned installations wherein the stray bodies have a tendency to come into collision with one part of the installation preferentially. For example, in the case of piping including valves, these preferential places may be elbows in the piping, the low points or again the valve bodies.
In the case of pressurized water nuclear reactors, the impact of stray bodies on certain parts of the primary circuit may be particularly dangerous since the flow speeds of the fluids are very high, of the order of 10-15 m per second. Such impacts may hence cause considerable damage in the reactor circuit.
This damage is all the more difficult to repair if the reactor has already operated and when repairs must be carried out in areas subject to radiation from ionizing particles.
It is therefore important to detect such stray bodies very rapidly, from the first impacts on the structure of the circuit, so as to be able in the control room to make the appropriate decisions to avoid serious damage in the reactor.
It has long been known how to detect the impact of two metal parts, using sound detectors positioned on the structure or microphones positioned in the air in the vicinity of the structure subject to the impacts. When acoustic waves produced by a metal on metal impact are detected by detectors or microphones, the emission received is transformed to emit a pulse-type signal.
Consideration has therefore been given to installing, in the vicinity of the areas where the probability of impact is high, sensors which can be positioned also in the vicinity of the parts of which the probability of breakage is high.
In fact, in industrial plants such as nuclear reactors, the sensor may very well respond to other stresses than those which correspond to the phenomenon of shock or of rupture which it is desired to monitor. Noises of very varied source, such as the noise accompanying the closing of the flap of a valve, the movement of the control rods or of other movable members may generate pulse-type acoustic signals which are observed at the terminals of the sensors and which can be taken for signals resulting from an impact between two metal parts in the area being surveyed.
Even using sensors such as acoustic emission piezoelectric sensors whose response frequencies are relatively high (above 50 kHz) and whose sensitivity area is smaller than, for example, those of accelerometers which take into account the low frequency components of the acoustic zones, it is not possible to discriminate effectively between the signals due to the impacts of the stray bodies and the signals due to other causes in other parts of the installation.
In particular, presently known devices do not enable a distinction to be made between a weak acoustic stress whose source is close and a greater stress produced at a great distance.
In the case of nuclear reactors, currently known devices do not permit distinction, for example, between the acoustic waves due to an impact on the bottom of a steam generator, for example, and the closing of a flap valve at very great distance from the area wherein the sensor is located. The exploitation of the recordings of the sensors by an electronic monitoring device including simple counting or recording means is hence not possible since it must take into account the shape of the signals and analyse them with the aid of an oscilloscope.
Moreover, an electrical interference may be observed by the monitoring circuit as an acoustic pulse, which is the cause of false alarms.